The long-term objective of the proposed work is to define the major features of the life-cycle of Babesia microti with reference to identifying conditions that affect infection of the tick vector and development of parasites infectious for the vertebrate host. In particular we shall: (1) Identify the stimuli inducing gamete formation. (2) Locate resting stage B. microti in immature I. dammini. (3) Describe the maturation of the resting stage. (4) Identify the route by which sporozoites leave the vector tick. (5) Explore alternatives to larval-nymphal transatadial passage. (6) Determine whether B. microti persists in nymphs following blood-feeding. (7) Identify factors controlling B. microti development in I. dammini. (8) Determine vector competence of I. muris, and (9) Describe morphogenesis of B. microti in vector ticks. This project applies experimental and electron microscopic techniques to describe and explain morphogenesis of an important human pathogen. The results will provide a basis for determining vector competence.